Epoxy systems consist of two components that can chemically react with each other to form a cured epoxy, which is a hard, duroplastic material. The first component is an epoxy resin and the second component is a curing agent, sometimes called a hardener. Epoxy resins are substances or mixtures which contain epoxide groups. The hardener can include compounds which are reactive to the epoxide groups of the epoxy resins.
The epoxy resins can be crosslinked, also referred to as curing, by the chemical reaction of the epoxide groups and the compounds of the hardener. This curing converts the epoxy resins, which have a relatively low molecular weight, into relatively high molecular weight materials by chemical addition of the compounds of the hardener. Additionally, the hardener can contribute to many of the properties of the cured epoxy.
Some of the hardeners, however, suffer from the disadvantage that they contain up to 50 weight percent free (alkyl)phenol and/or volatile organic compounds like benzyl alcohol. Volatile organic compounds are defined in various terms depending upon region. For example, in the European Union one definition of a volatile organic compound is any organic compound having an initial boiling point less than or equal to 250 degrees Celsius measured at a standard atmospheric pressure of 101.3 kilopascal.
In recent years due to environmental concerns and governmental regulations, there have been increased efforts made to develop curable epoxy systems which contain a minimum of volatile organic compounds and/or comply with governmental regulations.
As noted above, epoxy resins can be crosslinked in order to develop certain characteristics. Blushing can occur during the crosslinking. Blushing, sometimes also referred to as whitening, can occur when moisture, such as atmospheric water or water that originates from within a porous substrate together with atmospheric carbon dioxide, reacts with a curable composition having a hardener that includes an amine compound. Amine compounds on the surface of the curable composition can combine with the water and the carbon dioxide to form carbamates. The amine compounds, which were intended to react with the epoxide groups of the epoxy resins, are consumed and thus not all epoxy resins can crosslink during curing. Blushing can produce white patches or hazy effect portions in clear coatings. This can contribute to discoloration over time, and may cause lack of gloss in pigmented coatings. Furthermore, blushing can affect the coating performance and result in poor overcoatability. Poor overcoatability is the insufficient adhesion of a subsequent coating layer due to a surface energy modification associated with the blushing.